The present invention relates to a method for operating a combined gas turbine/steam turbine process in which the superimposed boosted gas turbine process is operated with compressed air and a combustible fluid, and the steam turbine process is operated via a fluidized bed furnace with a carbonaceous solid fuel, whereby the combustion in the fluidized bed furnace is performed with the oxygen-containing exhaust gases of the superimposed gas turbine process and electrical energy is generated with both turbine processes.
The exhaust gases of the superimposed gas turbine process is expended only to such an extent that the fluidized bed furnace may be operated as a pressurized fluidized bed. The flue gases of the pressurized fluidized bed furnace, after removal of dust, are expanded in a second gas turbine process and subsequently subjected to a heat exchange. The air for the superimposed gas turbine process is subjected to a two-stage compression with recooling between the compression stages with the aid of gas turbine processes, whereby the compression energy for the first stage is derived from the second gas turbine process and the compression energy for the second stage is derived from the superimposed gas turbine process.
Such a method is known from DE-A-32 24 577 in which the air subjected to the two-stage compression is heated via air heating surfaces that are arranged within the furnace chamber of the fluidized bed furnace. Thus, high-quality heat is transferred onto the compressed air which is subsequently released during the combustion of the solid fuel within the pressurized fluidized bed at approximately 850.degree. C. The portion of the combustion heat which has been used for heating up the compressed air, is not accessible to the steam generation for the steam turbine process but is introduced into the low pressure gas turbine process which, due to its limited turbine introduction temperature, has only a medium efficiency.
A method is known from DE-OS 36 12 888 in which a gas is used as the combustible fluid which is produced from a solid carbonaceous material by pressure gasification in a circulating fluidized bed. The gas produced by pressure gasification is expanded to a very low pressure stage, for example, 1.35 bar and is then fed into a circulating atmospheric fluidized bed. The total efficiency that can be achieved with the known method is 42%.
A method for operating a combined gas turbine/steam turbine process is also known from DE-OS 35 36 451 in which the compression of the combustion and fluidizing air necessary for the operation of the pressurized fluidized bed is achieved in two stages, the first stage being performed in a compressor of a turbo charger, and the second stage being performed in a compressor driven by a gas turbine. Between the two stages the air is intermediately cooled, in order to reduce the amount of compression work. Before entering the furnace chamber of the pressurized fluidized bed furnace, the air is preheated in a air preheating unit, which is operated with the partially expanded flue gas of the pressurized fluidized bed furnace that has been expanded in the gas turbine. In the air preheating unit heat is transferred to the compressed air from the flue gas which is still under a higher pressure. The air preheating unit has connected thereto in a down-stream position a further feed water preheating unit in which heat is also transferred to the feed water. The remaining energy within the flue gas is sufficient to operate the turbo charger of the first compressing stage. With the known method a net efficiency of 38% may be achieved. The combustion temperature of 850.degree. within the pressurized fluidized bed therefore sets a limit for the improvement of the efficiency as well as the specific work of the combined process with a pressurized fluidized bed.
It is therefore an object of the present invention to improve the efficiency of the known combined gas turbine/steam turbine process with a boosted superimposed gas turbine.